This invention relates to the preparation of aniline and diphenylamine. More particularly, it relates to an integrated process for the coproduction of aniline and diphenylamine from phenol and ammonia. Even more particularly, it relates to a novel process wherein an aniline plant and diphenylamine plant are integrated to obtain maximum economic and technical advantage.
Aniline is a commercial chemical of great industrial importance. Its uses are varied and include application as rubber accelerators, antioxidants, dyes, dye intermediates, drug intermediates, explosives and fuel. Conventionally, aniline is prepared by the reduction of nitrobenzene with iron filings or borings and 30% hydrochloric acid; by reaction of chlorobenzene with aqueous ammonia at 200.degree. C. and 800 p.s.i.; and by catalytic vapor phase reduction of nitrobenzene with hydrogen.
It is also known that high yields of aniline can be obtained from phenol by catalytic exchange of the hydroxy radical for the amino radical in the presence of ammonia. This reaction requires a solid catalyst, preferably an alumina or silica-alumina catalyst as is disclosed in U.S. Pat. No. 3,272,865, U.S. Pat. No. 3,578,714 and U.S. Pat. No. 3,860,650. Reaction conditions usually employed are those disclosed in the aforesaid patents and include temperatures between about 300.degree. C. and 600.degree. C. Reaction pressures are superatmospheric and are preferably above 7 atmospheres.
Diphenylamine is also a well-known industrial product with many uses, e.g. in the production of antioxidants for elastomers and in the manufacture of azo dyes. Diphenylamine has generally been prepared commercially by catalytically deammoniating or selfcondensing aniline according to the equation: EQU 2 C.sub.6 H.sub.5 NH.sub.2 .fwdarw.C.sub.6 H.sub.5 NHC.sub.6 H.sub.5 +NH.sub.3
as shown, for example in British Pat. No. 752,859 and in U.S. Pat. Nos. 3,071,619, 2,447,044, 2,645,662 and 3,944,613.
It is also known to produce diphenylamine by catalytically reacting aniline with phenol by the reaction EQU C.sub.6 H.sub.5 NH.sub.2 +C.sub.6 H.sub.5 OH.fwdarw.C.sub.6 H.sub.5 NHC.sub.6 H.sub.5 +H.sub.2 O.
Such a process is described, for example, in U.S. Pat. No. 2,824,137 which employs titanium catalysts, in British Pat. No. 1,541,153 which uses phosphoric acid as a catalyst, and in Japanese Patent Publication No. 75/07,061 which employs an alumina catalyst. Best results are obtained by reacting phenol and aniline in the presence of an acidic alumina-containing catalyst and especially alumina-silica catalysts containing 5-95% silica as disclosed in U.S. Pat. No. 3,860,650.
Heretofore, to produce diphenylamine from aniline and phenol, it has been necessary to first produce purified aniline and then react the aniline with phenol. To efficiently produce purified aniline it is necessary to maintain a substantial stoichiometric excess of ammonia with respect to phenol in the aniline reaction step. The excess of ammonia is conducive to a fast and selective reaction of ammonia with phenol. Substantial capital and processing expenditures are required, however, for the distillation separation of ammonia from the effluent of the aniline reactor and recycle of the excess ammonia to that reactor.
The process for preparing purified aniline further includes the distillation of the crude aniline reactor effluent from which ammonia and water have been removed, into an overhead aniline product and a bottoms product which comprises an azeotrope of aniline and phenol together with minor but significant amounts of byproduct diphenylamine. The latter byproduct stream is then distilled to separate a phenol-aniline overhead stream which is recycled to the aniline reactor and a bottoms stream containing diphenylamine.
The diphenylamine process has a number of features in common with the aniline process, among them, the following. Phenol is a reactant common to each process; the diphenylamine reaction, like the aniline reaction, is an equilibrium reaction which does not go to completion with the result that unreacted phenol is found in the effluent from both reactors; and diphenylamine can be recovered in purified form from the aniline plant as in the diphenylamine plant although in substantially smaller amounts.
These and other common features of the two processes have been found to lend themselves to an overall integration of the two processes with major and unexpected technical and cost advantages to both process plants.